Transformation of Amides into Highly Functionalized Triazolines - ACS

Research Article pubs.acs.org/acscatalysis

Transformation of Amides into Highly Functionalized Triazolines Tove Slagbrand,† Alexey Volkov,† Paz Trillo,† Fredrik Tinnis,*,† and Hans Adolfsson*,†,‡ †

Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden Department of Chemistry, Umeå University, SE-901 87 Umeå, Sweden

‡

S Supporting Information *

ABSTRACT: Triazoles and triazolines are important classes of heterocyclic compounds known to exhibit biological activity. Significant focus has been given to the development of synthetic approaches for the preparation of triazoles, and they are today easily obtainable through a large variety of protocols. The number of synthetic procedures for the formation of triazolines, on the other hand, is limited and further research in this field is required. The protocol presented here gives access to a broad scope of 1,4,5-substituted 1,2,3-triazolines through a one-pot transformation of carboxamides. The two-step procedure involves a Mo(CO)6-catalyzed reduction of tertiary amides to afford the corresponding enamines, followed by in situ cycloaddition of organic azides to form triazolines. The amide reduction is chemoselective and allows for a wide variety of functional groups such as esters, ketones, aldehydes, and imines to be tolerated. Furthermore, a modification of this one-pot procedure gives access to the corresponding triazoles. The chemically stable amide functionality is demonstrated to be an efficient synthetic handle for the formation of highly substituted triazolines or triazoles. KEYWORDS: triazolines, amides, enamines, chemoselective reduction, azides

■

INTRODUCTION Triazolines have been evaluated as potential anticonvulsant drugs and are also important intermediates in organic synthesis.1 Wolff and co-workers discovered that 1,2,3-triazolines could be prepared by a 1,3-dipolar addition reaction of olefins and azides in 1912.2 Huisgen and L’abbé have demonstrated that the reactivity of the alkenes in this type of cycloaddition is significantly influenced by the electronic properties of the double bond and reactions with electrondeficient alkenes required long reaction times of weeks or even months.3 Weinreb and co-workers later reported that the rate of cycloaddition could be increased substantially by employing high pressure (12 kbar), and a selection of 1,4,4-trisubstituted triazolines was prepared.4 Electron-rich olefins such as enamines are much more reactive and have been demonstrated to readily undergo a regiospecific 1,3-dipolar addition with azides to afford 1,2,3-triazolines under mild conditions (Figure 1a).5 Nevertheless, in these cases the enamines were prepared and isolated prior to use, which limited the overall yields and the scope of the target heterocycles. Only a few examples of in situ formation of enamines in azide cycloaddition reactions have been reported. Bianchetti et al. showed that enamines prepared from acetone and aliphatic amines reacted with aryl azides in a tandem reaction to yield triazoles, but only a few triazolines were isolated.6 Stradi and Pocar later employed aldehydes or ketones in combination with primary or secondary amines to form enamines in situ.7 A good number of triazolines were isolated in low to excellent yields; however, only 4nitrophenyl azide was used as the azide component (Figure 1b). Amine-catalyzed formation of enamines has also been employed in some cases for subsequent cycloaddition © 2017 American Chemical Society

Figure 1. 1,3-Dipolar cycloaddition of enamines and azides to yield triazolines or triazoles.

reactions.8 The use of an azide in this type of reaction only gives the saturated triazoline as an intermediate, yielding triazole as the final product after elimination of the amine catalyst (Figure 1c).9 An alternative route for the formation of Received: January 11, 2017 Published: January 20, 2017 1771

DOI: 10.1021/acscatal.7b00095 ACS Catal. 2017, 7, 1771−1775

Research Article

ACS Catalysis triazolines can be achieved through the cycloaddition reaction of diazo compounds with Schiff bases.10 Herein we demonstrate a mild and efficient one-pot transformation of stable amides that gives access to a wide scope of highly functionalized triazolines (Figure 1d). We also show that 1,2,3-triazoles, which normally are obtained by the Cu-catalyzed azide−alkyne cycloaddition reaction (the “click” reaction),11 can be generated using our protocol. The developed protocol involves the reduction of amides to enamines using catalytic amounts of Mo(CO)6 in combination with the inexpensive and stable silane 1,1,3,3-tetramethyldisiloxane (TMDS). An unprecedented chemoselectivity of this transformation is demonstrated and functional groups such as nitro, nitrile, ester, ketone, imine, and aldehyde were tolerated. The target heterocyclic compounds were obtained via in situ cyclization reactions of enamines with a wide scope of aromatic and aliphatic azides.

Scheme 1. One-Pot Procedure for the Formation of Triazolines from Amides

present from the start. It was observed that the reduction of the amide was retarded and progressed very slowly; however, some amount of triazoline was detected in the crude 1H NMR, suggesting that the catalyst is poisoned by the final product rather than the azide. The product inhibition of the catalyst was also demonstrated by employing 5 mol % of triazoline from the start in the reduction of amide, which resulted in a very low 1H NMR yield of the enamine in comparison to the standard reaction (see the Supporting Information). Initially, a substrate evaluation investigating the reduction of different amides into enamines was performed. High 1H NMR yields of the corresponding enamines derived from amides 1a− d were observed by employing catalytic amounts of Mo(CO)6 and TMDS as the hydride source. The subsequent cycloaddition using phenyl azide (3a) readily gave the triazolines 4aa−4da in high isolated yields (Scheme 2). Munk and co-workers previously reported the cycloaddition between azides and enamines to be regiospecific, and in a recent mechanistic study, the Houk group found evidence for the reactions to be concerted.5b,c The regiochemistry of the 1,4,5-substituted triazoline 4da as shown in Scheme 2 was

■

RESULTS AND DISCUSSION We have recently reported on a mild and chemoselective protocol for the hydrosilylation of tertiary amides into either aldehydes or amines (Figure 2).12 Enamine formation was

Scheme 2. Evaluation of Tertiary Amides in the Chemoselective Reduction and Subsequent Cycloaddition Reaction with Organic Azidesa Figure 2. Mo(CO)6-catalyzed chemoselective reduction of amides extended to include formation of enamines.

observed when benzylic and aliphatic amides were employed. Since only a limited amount of amide reduction protocols are known to afford enamines,13 we initiated an optimization of the conditions for the Mo(CO)6-catalyzed system to furnish these valuable compounds. Furthermore, we envisioned that a mild and chemoselective formation of enamines from highly stable carboxamides could be exploited in the synthesis of 1,2,3triazolines and that this approach would considerably broaden the scope and synthetic accessibility of these heterocycles. After thorough optimization of the conditions it was found that the catalytic loading of Mo(CO)6 can be decreased to 2 mol % and the amount of TMDS to 1.5 equiv, allowing for high conversion of these types of amides to the corresponding enamines. Surprisingly, the screening of different solvents revealed that the reduction of the amides could be performed in ethyl acetate14 (see the Supporting Information), which further demonstrates the high chemoselectivity of the system. With the optimized conditions for the enamine formation in hand, we decided to attempt the 1,3-dipolar addition reaction using phenyl azide 3a (Scheme 1). The enamine reaction was completed in 1 h, and by the subsequent addition of 1.5 equiv of 3a, compound 4aa could be isolated in 91% yield after 3 h reaction time. We then investigated if the transformation could be performed as a tandem process with all the components

a Isolated yields. Step 1: Mo(CO)6 (0.02 mmol), amide (1.0 mmol), dried ethyl acetate (0.5 mL, 2 M), TMDS (1.5 equiv), 65 °C for 1 h. Step 2: azide (1.5 equiv), 65 °C for 2 h. For information about the conditions for each substrate see Tables S2 and S3 in the Supporting Information. bDiastereomers were obtained in a 2:1 ratio in which the major compound was determined to have a cis relationship between the phenyl group and the piperidine moiety.

1772

DOI: 10.1021/acscatal.7b00095 ACS Catal. 2017, 7, 1771−1775

Research Article

ACS Catalysis confirmed by 1D NOE irradiation (see the Supporting Information), which is in line with previous reports. An αdisubstituted amide (1e) was evaluated in the enamine formation reaction, resulting in a mixture of cis and trans isomers. After the cycloaddition step the corresponding triazoline (4ea) was obtained as two diastereomers in a 2:1 ratio. The major diastereomer was separated and the relative stereochemistry was determined using 1H NMR NOE experiments. By irradiation of the signal of the CH3 group, an increase in intensity of the signal belonging to the adjacent proton was observed, indicating a cis relationship between these two groups. A few amide substrates were observed to be challenging to reduce, and only small amounts of the corresponding enamines 2j−l were detected by 1H NMR (Scheme 2). The enamine formation of purely aliphatic amides did not display a high selectivity, and some amount of amine was detected by 1H NMR spectroscopy. Thus, the propyl-substituted triazoline 4fa was obtained in a moderate yield of 66%. We were pleased to see that the high chemoselectivity of the amide reduction allowed for the presence of easily reducible functional groups such as esters, ketones, and imines, ultimately accessing triazolines 4ga−ia in 94%, 84%, and 88% yields, respectively. It was observed that aldehyde substituents on the amide were not affected by the reduction conditions but rather by the enamine being formed during the reaction. We hypothesized that the presence of an azide from the start would intercept the enamine, and although the tandem reaction was observed to be slow, we adopted this strategy for the p-aldehyde-substituted amide 1m (Scheme 3). To overcome the issue of catalyst

Scheme 4. Investigation of the Reactivities of Different Amides in Enamine and Triazoline Formation

Scheme 5. Scope Evaluation of (a) the Amine Part of the Amide Substrates and (b) Different Organic Azidesa

Scheme 3. Transformation of Aldehyde-Containing Amide in a Tandem Process

poisoning by triazoline, the loading of Mo(CO)6 had to be increased to 30 mol % and the reaction required 72 h to reach completion. Nevertheless, this demonstrates that the Mo(CO)6-catalyzed reduction of amides to enamines can even tolerate aldehydes and triazoline 4mb was isolated in 80% yield. We decided to evaluate how different amine parts affected the reactivity in the reduction step toward enamine and in the 1,3-cycloaddition reaction (Scheme 4). An aliquot of the Mo(CO)6-catalyzed reduction was taken and analyzed by 1H NMR after 15 min reaction time. Amides derived from both pyrrolidine and dimethylamine showed high conversion into the corresponding enamines, while the piperidine amide reacted more slowly (Scheme 4). The pyrrolidine-based enamine was proven to be the most reactive in the cycloaddition reaction, which is in line with previous studies.15 The dimethylamine- and piperidine-derived enamines were less reactive, and only about 15% of the corresponding triazolines were observed after 1 h reaction time. Amides derived from cyclic as well as noncyclic amines were evaluated in the transformation, and the corresponding triazolines were obtained in good to excellent yield (Scheme 5a). The N-methyl-N-phenyl amide 1s proved to be more difficult to reduce, and a mixture of enamine, amine, and

a Isolated yields. Step 1: Mo(CO)6 (0.02 mmol), amide (1.0 mmol), dry ethyl acetate (0.5 mL, 2 M), TMDS (1.5 equiv), 65 °C for 30 min. Step 2: azide (1.5 equiv) at 65 °C for 3 h. For information about conditions for each substrate see Tables S2 and S3 in the Supporting Information. bpiperidine substituted triazoline.

products of C−N bond cleavage were observed under the standard reaction conditions. The mechanism of Mo(CO)6catalyzed amide reduction was thoroughly investigated by Pannell and co-workers, demonstrating the formation of silylhemiaminal as an intermediate.16 It was envisioned that the presence of a catalytic amount of base would promote the collapse of the tetrahedral intermediate into enamine, yielding a cleaner reaction. Gratifyingly, the addition of triethylamine (10 mol %) to the reaction mixture resulted in the selective formation of the corresponding enamine 2s in >95% 1H NMR 1773

DOI: 10.1021/acscatal.7b00095 ACS Catal. 2017, 7, 1771−1775

Research Article

ACS Catalysis yield after 15 h. The more reactive p-CF3 phenyl azide 3b was used in the subsequent cycloaddition reaction, and the target triazoline 4sb was obtained in 82% isolated yield. Primary and secondary amides were also evaluated in the Mo(CO)6catalyzed reduction; however, the corresponding enamines were not observed. Next, the azide scope of the 1,3cycloaddition reaction was evaluated using pyrrolidine amide 1n in most of the cases (Scheme 5b). The one-pot system was shown to be compatible with a wide variety of azides substituted with functional groups such as aldimine (4nc), olefin (4nd), alkyne (4ne), iodo (4nj), cyano (4nk), nitro (4al), ester (4nr), and ketone (4ns). In the cyclization reaction with aldehyde-substituted azide 3t a high isolated yield of the corresponding triazoline 4nt could be obtained. This further demonstrates that an electron-poor azide reacts more readily with the enamine in comparison to the aldehyde. It was observed that benzyl azides reacted sluggishly at 65 °C, and employing the compound 3g containing both aromatic and benzylic azides resulted in the selective formation of triazoline 4ag in 78% yield with preservation of the benzylic azide. On the other hand, the p-CF3-substituted benzyl azide was demonstrated to be more reactive, which enabled access to the benzylated triazoline 4nh after a prolonged reaction time of 18 h. Heteroaryl azide and an aliphatic azide bearing electronwithdrawing groups could also be employed under mild reaction conditions, and the corresponding triazolines 4nf,ni were obtained in 88% and 79% yields, respectively. It was noticed that the cyclization reaction was highly influenced by electronic properties of the azides, and whereas the electron-deficient substrates reacted readily, the electronrich substrates required longer reaction times and higher reaction temperatures. Xie et al. recently reported 1,3cycloadditions of enamines employing a variety of perfluorinated aryl azides.15 The triazolines formed with these electrondeficient groups were unstable and rearranged spontaneously into the corresponding amidines. When we evaluated the combination of enamine and azide both bearing electronwithdrawing substituents, the corresponding triazole was obtained instead. Thus, the unsaturated heterocyclic compound 5ub was isolated in 58% yield under the standard reaction conditions (Scheme 6). Furthermore, forcing unreactive benzyl and aliphatic azides to undergo cycloaddition at higher reaction temperatures (80 °C) enabled access to the corresponding triazoles 5nu,nv in good to excellent yields in 24 and 65 h, respectively. Triazolines are known to be thermally labile,4 and the elevated temperature required for these substrates most likely promoted the elimination of the amine to form a triazole. Munk and co-workers showed that triazoles were obtainable from the corresponding triazolines by subjecting them to an excess of KOH/MeOH solution under reflux conditions.5a A minor screening of conditions was therefore performed in order to evaluate if triazoles could be accessed in a three-step one-pot fashion. It was found that the addition of catalytic amounts of base, after the completed triazoline reaction, initiated the amine elimination and pure triazole products could be obtained. Triazoles 5na,ag were isolated in good to high yields using this one-pot methodology (Scheme 6b). To demonstrate the versatility of the one-pot procedures for triazoline and triazole formation, we carried them out on a preparative scale (Scheme 7). Although we never experienced any issues, one should always be aware of the explosion risks associated with organic azides.17 By performing the reaction on

Scheme 6. (a) Direct Transformation of Amides into Triazoles and (b) Three-Step/One-Pot Procedure for Triazole Formationa

a

Isolated yields. Step 1: Mo(CO)6 (0.02 mmol), amide (1.0 mmol), dried ethyl acetate (0.5 mL, 2 M), TMDS (1.5 equiv), 65 °C for 30 min. Step 2: azide (1.5 equiv) at 65 °C, 3 h. For information on the conditions for each substrate see Table S4 in the Supporting Information.

a 5 mmol scale in a two-necked round-bottomed flask fitted with a condenser, we could obtain triazoline 4ns in 92% yield (1.53 g) (Scheme 7a). Nitriles were well tolerated in the Mo(CO)6-catalyzed amide reduction, and the reaction of 1v and p-cyanophenyl azide (3k) led directly to the formation of the corresponding unsaturated heterocycle and did not require the catalytic addition of base. Thus, triazole 5vk was obtained in 88% yield after washing the crude reaction mixture with pentane (Scheme 7b). Scheme 7. Large-Scale Transformation of Amides into (a) Triazoline and (b) Triazole

■

CONCLUSIONS To conclude, we have for the first time demonstrated a mild and efficient transformation of stable carboxamides into either triazolines or triazoles. The amides were reduced into enamines employing catalytic amounts of Mo(CO)6 in combination with TMDS as the hydride source using EtOAc as solvent. The reduction is characterized by an unprecedented level of chemoselectivity, and functional groups such as nitro, nitriles, esters, and imines were tolerated. Moreover, using the developed reduction protocol it was possible to form enamines containing ketone and aldehyde functionalities which cannot be accessed through classical condensation reaction. The enamines 1774

DOI: 10.1021/acscatal.7b00095 ACS Catal. 2017, 7, 1771−1775

Research Article

ACS Catalysis

(7) Stradi, R.; Pocar, R. Gazz. Chim. Ital. 1969, 99, 1131−1137. (8) (a) Xie, H.; Zu, L.; Oueis, H. R.; Li, H.; Wang, J.; Wang, W. Org. Lett. 2008, 10, 1923−1926. (b) Samanta, S.; Krause, J.; Mandal, T.; Zhao, C.-G. Org. Lett. 2007, 9, 2745−2748. (c) Juhl, K.; Jørgensen, K. A. Angew. Chem., Int. Ed. 2003, 42, 1498−1501. (9) (a) Thomas, J.; Jana, S.; John, J.; Liekens, S.; Dehaen, W. Chem. Commun. 2016, 52, 2885−2888. (b) Zhou, X.; Xu, X.; Liu, K.; Gao, H.; Wang, W.; Li, W. Eur. J. Org. Chem. 2016, 2016, 1886−1890. (c) Ramachary, D. B.; Shashank, A. B. Chem. - Eur. J. 2013, 19, 13175−13181. (d) Danence, L. J. T.; Gao, Y.; Li, M.; Huang, Y.; Wang, J. Chem. - Eur. J. 2011, 17, 3584−3587. (e) Belkheira, M.; El Abed, D.; Pons, J.-M.; Bressy, C. Chem. - Eur. J. 2011, 17, 12917− 12921. (10) (a) Ahamad, S.; Kant, R.; Mohanan, K. Org. Lett. 2016, 18, 280−283. (b) Kuznetsov; Gulevich, A. V.; Wink, D. J.; Gevorgyan, V. Angew. Chem., Int. Ed. 2014, 53, 9021−9025. (c) Bartnik, R.; Leśniak, S.; Wasiak, P. Tetrahedron Lett. 2004, 45, 7301−7302. (d) Kadaba, P. K. Tetrahedron 1969, 25, 3053−3066. (e) Kadaba, P. K. Tetrahedron 1966, 22, 2453−2460. (11) (a) Meldal, M.; Tornøe, C. W. Chem. Rev. 2008, 108, 2952− 3015 and references therein. (b) Hein, J. E.; Tripp, J. C.; Krasnova, L. B.; Sharpless, K. B.; Fokin, V. V. Angew. Chem., Int. Ed. 2009, 48, 8018−8021. (c) Worrell, B. T.; Ellery, S. P.; Fokin, V. V. Angew. Chem., Int. Ed. 2013, 52, 13037−13041. (12) (a) Volkov, A.; Tinnis, F.; Slagbrand, T.; Pershagen, I.; Adolfsson, H. Chem. Commun. 2014, 50, 14508−14511. (b) Tinnis, F.; Volkov, A.; Slagbrand, T.; Adolfsson, H. Angew. Chem., Int. Ed. 2016, 55, 4562−4566. (13) (a) Bower, S.; Kreutzer, K. A.; Buchwald, S. L. Angew. Chem., Int. Ed. Engl. 1996, 35, 1515−1516. (b) Motoyama, Y.; Aoki, M.; Takaoka, N.; Aoto, R.; Nagashima, H. Chem. Commun. 2009, 11, 1574−1576. (c) Volkov, A.; Tinnis, F.; Adolfsson, H. Org. Lett. 2014, 16, 680−683. (d) Tahara, A.; Miyamoto, Y.; Aoto, R.; Shigeta, K.; Une, Y.; Sunada, Y.; Motoyama, Y.; Nagashima, H. Organometallics 2015, 34, 4895− 4907. (14) (a) Alfonsi, K.; Colberg, J.; Dunn, P. J.; Fevig, T.; Jennings, S.; Johnson, T. A.; Kleine, H. P.; Knight, C.; Nagy, M. A.; Perry, D. A.; Stefaniak, M. Green Chem. 2008, 10, 31−36. (b) Prat, D.; Wells, A.; Hayler, J.; Sneddon, H.; McElroy, C. R.; Abou-Shehada, S.; Dunn, P. J. Green Chem. 2016, 18, 288−296. (15) Xie, S.; Lopez, S. A.; Ramström, O.; Yan, M.; Houk, K. N. J. Am. Chem. Soc. 2015, 137, 2958−2966. (16) Arias-Ugarte, R.; Sharma, H. K.; Morris, A. L. C.; Pannell, K. H. J. Am. Chem. Soc. 2012, 134, 848−851. (17) Bräse, S.; Gil, C.; Knepper, K.; Zimmerman, V. Angew. Chem., Int. Ed. 2005, 44, 5188−5240.

were subjected to organic azides to undergo 1,3-cycloaddition reactions in a one-pot procedure to yield the corresponding triazolines. The preparation of these heterocyclic compounds from amides is a new concept that should be found useful, and a broad scope of the triazolines was obtained using this methodology. The direct formation of triazoles was observed for certain substrates. Alternatively, triazoles were generated from the triazolines by the addition of a catalytic amount of base in a three-step one-pot procedure. The attractive properties of this system along with the scalability and low cost/high stability of TMDS should make this method applicable in both academic and industrial settings. Due to the mildness and high level of chemoselectivity it can easily be envisioned that amides could serve as stable protecting groups, which can be transformed into triazolines or triazoles at a late stage of synthesis.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.7b00095. Experimental data and compound characterization (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail for F.T.: [email protected]. *E-mail for H.A.: [email protected]. ORCID

Fredrik Tinnis: 0000-0003-1271-4601 Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS The authors acknowledge the K. A. Wallenberg Foundation and the Swedish Research Council for financial support. REFERENCES

(1) Examples of triazolines with biological activity: (a) Chow, A. H. L.; Chiu, F. C. K.; Damani, L. A.; Kadaba, P. K. Pharm. Res. 1996, 13, 179−182. (b) Kadaba, P. K.; Stevenson, P. J.; P-Nnane, I.; Damani, L. A. Bioorg. Med. Chem. 1996, 4, 165−178. (c) Kadaba, P. K. Curr. Med. Chem. 2003, 10, 2081−2108. (d) Dürüst, Y.; Karakuş, H.; Kaiser, M.; Tasdemir, D. Eur. J. Med. Chem. 2012, 48, 296−304. Examples of triazolines as intermediates: (e) Xie, S.; Zhang, Y.; Ramström, O.; Yan, M. Chem. Sci. 2016, 7, 713−718. (f) de Loera, D.; Stopin, A.; GarciaGaribay, M. A. J. Am. Chem. Soc. 2013, 135, 6626−6632. For excellent reviews about 1,2,3-triazolines, see: (g) Kadaba, P. K.; Stanovik, B.; Tišler, M. Adv. Heterocycl. Chem. 1984, 37, 217−349. (h) Padwa, A. 1,3-Dipolar Cycloaddition Chemistry; Wiley: New York, 1984; Vol. 1. (2) Wolff, L. Justus Liebigs Ann. Chem. 1912, 394 (23), 59−68. (3) (a) Huisgen, R.; Szeimies, G.; Mobius, L. Chem. Ber. 1966, 99, 475−490. (b) Broeckx, W.; Overbergh, N.; Samyn, C.; Smets, G.; L’abbé, G. Tetrahedron 1971, 27, 3527−3534. (4) Anderson, G. T.; Henry, J. R.; Weinreb, S. M. J. Org. Chem. 1991, 56, 6946−6948. (5) (a) Munk, M. E.; Kim, Y. K. J. Am. Chem. Soc. 1964, 86, 2213− 2217. (b) Meilahn, M. K.; Cox, B.; Munk, M. E. J. Org. Chem. 1975, 40, 819−824. (c) Lopez, S. A.; Munk, M. E.; Houk, K. N. J. Org. Chem. 2013, 78, 1576−1582. (d) Nomura, Y.; Takeughi, Y.; Tomoda, S.; Ito, M. M. Bull. Chem. Soc. Jpn. 1981, 54, 261−266. (e) L’abbé, G. Chem. Rev. 1969, 69, 345−363 and references therein. (6) Bianchetti, G.; Pocar, D.; Croce, P. D.; Stradi, R. Gazz. Chim. Ital. 1967, 97, 304−320. 1775

DOI: 10.1021/acscatal.7b00095 ACS Catal. 2017, 7, 1771−1775